-Corpse looting. If the attacking player wastes his units, and hundreds of them die on the walls, or near the wall, defender would get his peasants looting the corpses - and income from that is huuuge.
-Player`s inside the castle would still have access to resources, yes it will be significantly less, but it`ll give defender a chance to survive until his Allies can help, or he can prepare to counter attack.
-Since wood and stone is hard to come-by while under siege, defending player will mostly be able to build infantry/cavalry force. Which in turn will force him to ride out and take the battle outside.

The problem I see with corpse looting is that unless the battle took place in one player's base, it's going to be the winner of that battle that gets the benefit of the looting. This may contribute, rather than erode, the slippery slope and make it harder to make a comeback. This has always been my gripe with the feature. The other two points still don't change the fact that the besieged player is at a massive resource disadvantage. Unless your mechanics are very different from any RTS I've played before, a massive resource advantage is usually automatic victory by attrician.

That said, I haven't played DoF, so I can't say if things do work differently here. I'd honestly love to see battles turning around.

Well, any victory on "no man's land" will give corpse looting privileges to the victorious army (which concerns me a bit too for the reasons Darwin stated) but I expect the defending player will have dibs on any carcases that end up in the shadow his stronghold walls. So, if an attacker is careless he might end up "supplying" the defender with resources. One potential problem I see is if the defender ends up pinned inside the castle unable to loot before the next wave arrives. This depends on whether army camps can train new soldiers (I read ships will be able to do it) or does the attacker have to bring his units all the way from his stronghold.

It might be nice if any such non-stronghold "forward bases" that are able to build units did it at a slower rate than the stronghold which would allow the defender to recover a bit faster (if he has enough resources of course) than the attacker.

But yeah, don't change corpse looting until we've actually played the game.

As I see it, army camps should require some sort of supply routes so they are vulnerable to counter-harass. I think it's important for DoF to break up the "decisive force" strategy (putting 90% of your units into one group and attacking with it) which has dominated pretty much every RTS ever made and encourage "divide and conquer" strategies which have almost never worked out well in the RTS setting. It would be useless keeping all your troops in one group if their supply routes just get cut off, after all.

I agree with what Darvin said about adding supplies. But would they need to consume resources such as gold and food? or just be there? what consequences for the troops without supplies? do they lose health, maybe stamina? Something else is maybe allow forward bases to make units normally but you have to send the supplies to make the troops via convoy? An incentive for this could be that supply wagons are faster or use less supplies then simply marching an endless line of troops to the defenders stronghold from yours. This would allow the defender or allies to cut off the attackers supply route ending immediate reinforcements and starving the army.